FIG. 1 illustrates a high-level LTE network architecture where control plane and user plane traffic are separated. In this architecture, a core network, referred to as an Evolved Packet Core (EPC), may be composed of control and user-plane entities. The main control plane entity is referred to as a Mobility Management Entity (MME), while user plane traffic is handled by a Serving Gateway (SGW) and a Packet Data Network (PDN) Gateway (PDN GW or PGW). The PDN GW is the interface between the LTE networking subsystem and IP networks, including the public Internet, and Internet Protocol Multimedia Subsystem (IMS) services 130 that may be deployed within an Operator Core. FIG. 1 further illustrates an MME Pool 100 as comprising a plurality of MMEs, an SGW Pool 110 as comprising a plurality of SGWs, and a PDN GW Pool 120 comprising a plurality of PDN GWs. It should be noted that the number of MMEs, SGWs, and/or PDN GWs may be varied in accordance with different embodiments.
User authentication is handled through a Home Subscriber Server (HSS) 140, which is analogous to a home location register (HLR) in third generation (3G)/Global System for Mobile Communications (GSM) systems. An entity referred to as a Policy and Charging Rules Function (PCRF) 150 is responsible for tying quality of service (QoS) policies and billing mechanisms into a single cohesive framework.
Base stations 160a and 106b are referred to as evolved Node Bs (eNBs), and are linked to MMEs, such as those in MME Pool 100, using an S1-MME (also known as S1-C) interface. The eNBs 160a and 106b are linked to the SGWs, such as those in SGW Pool 110, using an S1-U interface. Such an S1-Flex mechanism (creating pools of MMEs and SGWs and allow eNBs to be connected to MMEs and SGWs in a pool) allows an eNB, e.g., eNB 160a or 106b, to connect to a pool of MMEs 100 and a pool of SGWs 110 for load balancing purposes.
FIG. 1 further illustrates various interface connections between the aforementioned LTE elements, i.e., S6a, Gx, S5/S8 and S11 interface connections. Different users served by a single eNB may be associated with different MMEs and SGWs. An eNB is roughly analogous to the combination of a node B and a radio network controller (RNC) in Universal Mobile Telecommunications System (UMTS) networks. A substantial difference from 3GPP UMTS systems is the flattened LTE architecture, which also utilizes a direct X2 interface between eNBs. Handover is initiated and handled directly between eNBs in the LTE architecture with the MMEs only playing an auxiliary role for user equipment (UE) tracking purposes.